#include<bits/stdc++.h>
#define ll long long
#define vi vector<int>
#define vll vector<ll>
#define eb emplace_back
#define pb push_back
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
#define sz(v) ((int)v.size())
#define pii pair<int,int>
#define pll pair<ll,ll>
#define fi first
#define se second
#define umap unordered_map
#define uset unordered_set
#define mset multiset
#define ui unsigned int
#define ull unsigned ll
#define i128 __int128
using namespace std;

namespace qiqi {
	const int V = 1e5+1;
	const i128 inf = 1e36;
	using B = bitset<V>;
	void main() {
		int n,m;
		cin >> n >> m;
		vi a(n);
		for (int i = 0; i < n; i++) {
			cin >> a[i];
		}
		sort(all(a));
		i128 ans = inf;
		auto calc = [&](i128 a,i128 b,i128 c) {
			i128 p = a+b+c;
			a *= 2; b *= 2; c *= 2;
			return p*(p-a)*(p-b)*(p-c);
		};
		for (int i = 0; i+1 < n; i++) {
			{
				int p = upper_bound(all(a),a[i+1]-a[i])-a.begin();
				if (p < i) {
					ans = min(ans,calc(a[p],a[i],a[i+1]));
				}
			}
			if (a[i] == a[i+1]) {
				int p = lower_bound(all(a),a[i]+a[i+1])-a.begin()-1;
				if (p > i+1) {
					ans = min(ans,calc(a[p],a[i],a[i+1]));
				}
			}
		}
		B p,q;
		for (int i = 0; i < n; i++) {
			p.set(a[i]); q.set(m-a[i]);
		}
		for (int i = 1; i <= 2*m; i++) {
			int x;
			if (i < m) {
				x = (p&(q>>(m-i)))._Find_first();
			}
			else {
				x = (p&(q<<(i-m)))._Find_first();
			}
			if (x*2 < i) {
				int y = i-x,z = *(lower_bound(all(a),i)-1);
				if (z > y) {
					ans = min(ans,calc(x,y,z));
				}
			}
		}
		if (ans == inf) {
			cout << -1 << '\n';
			return;
		}
		cout << sqrt((double)ans/16) << '\n';
	}
}

int main() {
//	clock_t st = clock();
//	freopen("test.in","r",stdin);
//	freopen("test.out","w",stdout);
	ios::sync_with_stdio(false);
	cin.tie(nullptr);

	cout << fixed << setprecision(15);

	int T = 1;
	cin >> T;
	while (T--) {
		qiqi::main();
	}

//	cout << (double)(clock()-st)/CLOCKS_PER_SEC;
	return 0;
}